Boston Celtics rookie forward Daniel Theis is expected to miss the remainder of the season after tearing his left meniscus, coach Brad Stevens said Monday. Theis suffered the injury on a knee-to-knee collision in the final moments of Sunday's loss to the Indiana Pacers. It's the latest hit for a Celtics team battling a rash of injuries.

"When he got buckled back [against Oladipo], I guess he tore a meniscus. So he is out for the foreseeable future. Most likely out all season," said Stevens. "It's too bad. He's had an incredible rookie year. He's obviously down and he's weighing surgical options right now and we'll go from there."

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After he was ruled out of Sunday's loss with left knee soreness, Celtics All-Star Kyrie Irving hinted at opting for an extended period of rest to rehabilitate the knee.

The Celtics played the second half of Sunday's game without four of their five opening-night starters after Kyrie Irving was forced out due to knee soreness. Irving is expected to sit out an extended stretch to combat lingering soreness, while both Al Horford (illness) and Jaylen Brown (concussion) are currently sidelined. Boston lost forward Gordon Hayward to a fractured ankle just minutes into their opening-night game against the Cleveland Cavaliers.

Stevens said Boston needs its youngest players to step up.

"We are equipped and we will battle on, I think is the way you gotta look at it," said Stevens. "I've never been in a season that's been like this. Obviously it started right out of the gate this way with Gordon. But, the rest of the year, we've dealt with a what would be small, week-to-week or couple-weeks-out type things. This is obviously a big hit with Theis' injury, no question."

Stevens said Monday that there was no update on Irving. Horford missed practice while still ill. Brown is expected to miss at least this week while working his way through the NBA's return-to-action concussion protocol.

The Celtics can lean heavier on backup big man Greg Monroe in the absence of Theis, but Stevens said rookies Semi Ojeleye and Guerschon Yabusele could see more playing time as well. The 6-foot-9 Theis, signed out of Germany this ofseason to add big-man depth, was averaging 5.3 points and 4.3 rebounds in 63 games this season. He had emerged as a key bench contributor for a Boston second unit that had started to play its best basketball recently.

"Just incredible reliability and energy.... You really feel for [Theis]," said Stevens. "I think that he came in with knowing he could play but maybe not even expecting to have the kind of season he had. And he didn't change that when we brought Greg in and we mixed up minutes a little bit here and there. ... We're going to miss him on the court but he'll have a full recovery and he'll be able to get back better than ever when he steps back out on the court."